The present invention relates to masks used in the production of semiconductor integrated circuits (IC's). More specifically, the present invention relates to mask structures and production methods for improving image resolution and depth of focus, and reducing image shortening effects, in photolithographic IC production (i.e., optical photofabrication).
Today, most semiconductor integrated circuits are formed utilizing optical photofabrication techniques. This typically involves the controlled projection of ultraviolet (UV) light through a mask (i.e., reticle) and onto a layer of light-sensitive resist material deposited on a semiconductor wafer. The mask typically embodies a light transmissive substrate with a layer of light blocking material defining a pattern of circuit features to be transferred to the resist coated wafer. If a negative acting resist is used, then the projected exposure light passing through the mask will cause the exposed areas of the resist layer to undergo polymerization and cross-linking resulting in an increased molecular weight. In a subsequent development step, unexposed portions of the resist layer will wash off with the developer, leaving a pattern of resist material constituting a reverse or negative image of the mask pattern. Alternatively, if a positive acting resist is used, the exposure light passed through the mask will cause the exposed portions of the resist layer to become soluble to the developer, such that the exposed resist layer portions will wash away in the development step, leaving a pattern of resist material corresponding directly to the mask pattern. In both cases, the remaining resist will serve to define a pattern of exposed semiconductor material that will undergo subsequent processing steps (e.g., etching and deposition) for forming the desired semiconductor devices.
The formation of circuit pattern features in the sub-micron range requires that a commensurate degree of resolution be obtained in the exposure step. Higher numerical light apertures and shorter light wavelengths (e.g., deep UV range) yield higher resolution, but at the expense of depth of focus. It is critical to increase as much as possible the depth of focus of the projected image. Typically, exposure light will be required to pass through relatively substantial resist material thicknesses, and it is important that the mask pattern be accurately projected throughout the depth of the resist material. Additionally, an increased depth of focus will minimize the adverse effects of slight deviations of the exposure tool from a best focus position (defocus conditions). Even the most precise photofabrication equipment cannot guarantee that sub-micron range deviations from a best focus position will not occur.
Recently, phase-shift masking techniques have been developed which significantly increase resolution for a given depth of focus. Phase-shifting masks (PSM's) are distinguished from conventional photolithographic masks by the employment of selectively placed mask pattern materials allowing the transmission of exposure light with a phase-shift of .pi. (180.degree.). First pioneered in the early 1980's, such techniques holds great promise for extending the limits of conventional photolithography to the production of circuit features as small as 0.25 .mu.m, and perhaps smaller. The 180.degree. phase difference created at the mask pattern edges sets up an interference effect that significantly enhances edge contrast, resulting in higher resolution and greater depth of focus (as compared to the conventional binary intensity masks utilizing only an opaque mask pattern material, e.g., chrome). Advantageously, the technique can be employed utilizing conventional photolithographic stepper optics and resist techniques.
Numerous PSM techniques have been developed. These include alternating, subresolution, rim, and attenuated phase-shifting techniques. See generally, C. Harper et al., Electronic Materials & Processes Handbook, 2d ed., 1994, .sctn. 10.4, pp. 10.33-10.39. Of these, attenuated phase-shifting techniques are among the most versatile, since they can be applied to any arbitrary mask pattern. In attenuated PSM's, a single slightly transmissive (halftone) absorber providing a phase-shift of 180.degree. can take the place of the conventional opaque, e.g., chrome, layer of mask pattern material. Originally, halftone materials were formed of two layers: a transmittance controlling layer and a phase controlling layer. More recently, advantages have been realized through the use of single layer materials developed to perform the dual function of controlling light transmittance and phase-shift. As reported in Ito et al., Optimization of Optical Properties for Single-layer Halftone Masks, SPIE Vol. 2197, p.99, January 1994 (hereby incorporated by reference in its entirety), one such material comprises SiNx, wherein the composition ratio is controlled by changing the amount of flowing nitrogen.
Although attenuated PSM's have proven to be one of the most useful techniques for applying actual device patterns with high resolution (see, e.g., K. Hashimoto et al., The Application of Deep UV Phase Shifted-Single layer Halftone reticles to 256 Mbit Dynamic Random Access Memory Cell Patterns, Jpn. J. Appl. Phys. Vol. 33 (1994) pp. 6823-6830, hereby incorporated by reference in its entirety), new techniques are required to provide even greater resolution so as to allow feature sizes at and below 0.25 .mu.m to be consistently produced with a low defect rate. Moreover, attenuated PSM's have not eliminated the problem of image shortening effects.
Image shortening is a phenomena that reduces the attainable whole resolution. With certain feature shapes, such as elongated holes used, e.g., to provide storage node, isolation and some contact hole levels in DRAM patterns, a slight defocus will result in a substantial shortening of the hole images projected onto the underlying wafer. This results because, particularly in a defocus condition, e.g., .+-.1.0 .mu.m, image intensity and contrast tend to decrease considerably toward the ends of the holes. This is illustrated by the simulated image intensity contour plots of FIG. 1B, for a conventional attenuated PSM.
Accordingly, there is a need for semiconductor photofabrication mask structures that will provide increased whole resolution and depth of focus, and which will minimize image shortening effects. There is also a need for efficient methods of producing such masks.